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We have known for at least a decade that hundreds of animal species  including birds, reptiles, mollusks and, of course, humans  engage in same-gender sexual acts. But no one is quite sure why. After all, same-sex couplings don't usually result in offspring. (I say usually because when male marine snails pair with other males, one partner conveniently changes sex, allowing for reproduction.) Evolutionarily speaking, homosexuality should have disappeared long ago.

A yearlong study just completed at the University of California at Riverside offers several fascinating competing theories about why same-gender sexual behavior has endured. And although it's gay-pride month  and the 40th anniversary of the Stonewall riots that sparked the gay-rights movement  not all the theories will give same-gender-loving humans a reason to celebrate. (See the top 10 animal stories of 2007.)

One particularly charged finding is that in most species besides humans, same-gender pairings rarely lead to lifelong relationships. In other words, when one attractive bonobo male eyes another in a lovely patch of Congo swamp forest, they occasionally kiss and then move on to other oral pleasures, but they don't bother anyone afterward about trying to legalize their right to an open-banana-bar ceremony. In fact, they are likely to move on to girl bonobos: most animals that engage in same-gender sex acts do so only when an opposite-sex partner is unavailable.

And yet the study's authors, Nathan Bailey and Marlene Zuk of UC Riverside's biology department, report some exceptions, like the laysan albatross. Last year, researchers studying a Hawaiian colony of albatrosses found that nearly a third of all the couples involved two females who courted and then shared parenting responsibilities. (Albatrosses don't have U-Hauls, so no lesbian jokes, please.) Male chinstrap penguins also form long-term relationships, at least in captivity. And some male bighorn sheep will mount females only after the females adopt male-like behaviors. (Watch a gay marriage wedding video.)

What explains all these variances? Here are some hypotheses I collected from Bailey and Zuk's paper as well as from some of their original sources:

1. The boys-in-the-locker-room theory. Any guy who played sports in high school knows that homoerotic jokes and towel-snapping are an underlying part of the subculture. Similarly, male bottlenose dolphins use same-sex sexual behavior to maintain and strengthen their social relationships  although dolphins are far more explicit about their homosexual play, regularly mounting one another and (hide the kids' ears here) sticking their noses into certain boy-dolphin parts. (Very regularly: roughly half of male dolphin sex occurs with other males.) Among bonobos, same-sex sexual behavior is also thought to ease social tension and facilitate reconciliation. And among garter snakes, male-on-male contact may allow some solitary males to thermoregulate and, therefore, survive.

2. The emasculation theory. Some male animals might mount other males as a way of denying them access to the ladies. For instance, as the Journal of Natural Historynoted in 2006, male dung flies often must compete violently to impregnate females. In those situations, "the most sensible strategy for beating a competitor in the race to an arriving female would be to mount him and remain in situ for as long as possible." Then, when the lady dung fly finally sails by, the aggressor male can pull himself out from the dominated male and  because he is on top  get above to the female faster.

3. The "oops" theory. Among insects, same-sex sexual behavior is usually a case of mistaken identity. Male fruit flies, for instance, may romance other males because they lack a gene that enables them to distinguish between sexes. Even more surprising, male toads can't tell the difference between girl toads and boy toads, so males will routinely embrace other males, although the subordinate ones are equipped with a call that quickly results in the dominant male releasing. In other species, the "straight" males get tricked by other wily straight males who dress in animal drag: male goodeid fish, for instance, sometimes have a black spot that resembles a spot that females get when pregnant. Dominant males then court them rather than fight with them. While the dominant guys are busy courting the subordinate, ladylike fish, the latter are able to "sneak copulations with females," as Bailey and Zuk write. I'm going to dub this the Hugh Grant Theory: it's not always the most masculine guy who gets the most girls.

4. The let's-see-how-this-thing-works theory. Younger animals (particularly males, and including humans) sometimes engage in same-sex sexual behavior as practice, which may improve their reproductive success when they are ready for a heterosexual relationship later. Fruit flies who experiment with other members of the same sex as youngsters may have more baby fruit flies later on than those who don't experiment.

5. The two-plus-one theory. Among flour beetles, males routinely force themselves on other males. According to Bailey and Zuk, there's some evidence that sperm deposited during this male beetle rape is sometimes transferred to a female later on, increasing the chances that she will have offspring.

What all these theories have in common is that same-sex sexual activity is either an accident or a quirky genetic method of helping males impregnate females. Which raises the evolutionary question of why men and women who are exclusive gay and lesbian exist. One answer is that exclusive gays and lesbians are a relatively new creation: the concept of exclusive homosexuality barely existed before modernity; even a century ago, most same-sex-attracted men and women got married and had kids. (Read "Do Monkeys Pay for Sex?")

As Bailey, Zuk and many others have pointed out, no one has offered an adequate evolutionary explanation for the relatively recent development of exclusive homosexuality among humans. In January, the journal Evolution and Human Behavior published a paper exploring the idea that certain alleles increase the likelihood of homosexuality by blocking the effect of androgens during fetal development. Having all those alleles hampers the masculinization of some parts of the brain that affect personality, making you gay, the theory goes. Brothers of gay men who have only some of the alleles would turn out straight but less aggressive than typical guys. And because those brothers exhibit less psychopathology, they would attract more women and therefore have more kids. It was a provocative theory, but it turned out not to be proved: gay men's brothers don't actually have more kids than straight men's brothers do.

So we're stuck at square one. As the 40th anniversary of Stonewall approaches, the question that Alan Miller and Satoshi Kanazawa ask in their 2007 book about evolutionary psychology, Why Beautiful People Have More Daughters, has never been more relevant: Will "the liberation of homosexuals, which allows them to come out of the closet and not pretend to be straight" actually turn out to "contribute to the end of homosexuality?" We may not know for a thousand years, but it's a great question.